Conventionally, as the membrane cartridge of this type, for example as shown in FIG. 27, a filtration membrane 3 is arranged on the surface of a filtration plate 2 having a square flat plate shape and the peripheral portion of the filtration membrane 3 is welded over the entire periphery of the surface of the filtration plate 2. The welded portion 12 is formed in a position slightly inward from a peripheral edge 3a of the filtration membrane 3. A range from the welded portion 12 of the filtration membrane 3 to the peripheral edge 3a is an un-joined portion 13 that is not welded.
A water intake nozzle 5 for sucking treated water obtained by membrane filtration is provided at the upper end of the filtration plate 2. A membrane cartridge 4 having the configuration as mentioned above is described in, for example, Japanese Patent No. 3010979. The membrane cartridge 4 has, in addition to the welded portion 12, intermittent welded portions formed by intermittently welding the peripheral edge 3a of the filtration membrane 3.
FIG. 28 shows a submerged membrane separator 6 including the membrane cartridge 4. This submerged membrane separator 6 is submerged in a treatment tank filled with activated sludge and separates activated sludge and treated water. A plurality of the membrane cartridges 4 are detachably stored in a box-type casing 7 opened at the top and the bottom in a state in which the membrane cartridges 4 are arrayed in parallel at predetermined intervals. An air diffuser 8 is provided below the membrane cartridges 4.
A water collecting pipe 10 for collecting the treated water is provided on one side of an upper opening 9 of the casing 7. The water collecting pipe 10 and water intake nozzles 5 of the membrane cartridges 4 are connected via connection pipes 11.
Consequently, during filtration operation, the inner sides of the membrane cartridges 4 are decompressed while air diffusion is performed from the air diffuser 8, whereby sludge or the like in liquid to be treated is caught by filtration membranes 3 of the membrane cartridges 4. Permeate permeating through the filtration membranes 3 and flowing into the inner sides of the cartridges 4 is discharged as treated water 14 to the water collecting pipe 10 from the water intake nozzles 5 through the connection pipes 11
In this case, upflow is caused between the membrane cartridges 4 by air bubbles of the air diffused from the air diffuser 8. The matter adhering to the membrane surface of the membrane cartridges 4 is removed by this upflow.
In some case, for example, when properties of the liquid to be treated are deteriorated, the sludge clogs in spaces between the membrane cartridges 4 and the membrane surfaces of the membrane cartridges 4 are blocked. When such blocking of the membrane surfaces occurs, after stopping the air diffusion by the air diffuser 8, stopping the filtration operation is stopped, and draining the liquid to be treated in the treatment tank, as shown in FIG. 29, an operator jets high-pressure cleaning liquid 15 to the spaces between the membrane cartridges 4 from the upper opening 9 of the casing 7. According to the jetting of the cleaning liquid 15, the sludge clogging the spaces between the membrane cartridges 4 is smashed and removed and the membrane surfaces of the membrane cartridges 4 are cleaned.